If there is only one sign of charge, how do you get neutral
pieces of paper?
That was probably intended as a rhetorical question, but
the answer is not as obvious as you might think.
As a humble example, consider masking tape. It sticks to
things; it never repels them. It can be "neutralized"
by folding it double.
If you want a more physics example, consider the atom--atom
attraction in liquid or solid xenon. The van der Waals
interaction is attractive for all distances outside some
small core.
From this we conclude that the mere observation that some
objects are mutually attracted, while others are not, does
not in principle suffice to show that there are two signs
involved.
Of course in the particular case of rubbing a balloon
and picking up bits of paper, we know electrical charge
is involved, and we know there are two signs of charge,
but this knowledge is based on *other* experiments.
The experiment that exhibits electrostatic repulsion is
easy to do, but somebody has to do it.
In general, the question of
-- What do you know?
is very different from
-- How do you know?
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